2.1. Hypothalamic Hormones
A neuroendocrine pathway by which the central nervous system communicates with the anterior pituitary is by way of the hypothalamic-hypophyseal portal systems. A number of small peptide (i.e., 3-44 amino acids) hormones, known as hypophysiotropic hormones are produced by cells of the hypothalamus in extremely small amounts. Each of these hormones has a different specific function, but overall, most are stimulatory to certain cells of the pituitary, which cause the release of specific anterior pituitary hormones. Among the well-known hypophysiotropic hormones, corticotropic releasing hormones stimulates release of adrenocorticotropic hormone (ACTH) and .beta.-endorphin; gonadotropin-releasing hormone (GnRH; also known as luteinizing hormone releasing hormone, LHRH) stimulates secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH); growth hormone releasing hormone (GHRH) stimulates growth hormone secretion; and thyrotropin releasing hormone (TRH) stimulates release of thyroid stimulating hormone (TSH) and prolactin. Some of these hypophysiotropic factors, however, are actually inhibitory; for example, somatotropin release inhibiting factor (somatostatin) inhibits secretion of growth hormone and TSH, and dopamine (a catecholamine) inhibits prolactin secretion.
A number of less well characterized, but apparently related "hormones" have also been described, many of which are associated with the digestive glands rather than the hypothalamus. One group of such peptides is the secretin-glucagon family of which vasoactive intestinal peptide (VIP) is a member. This family includes several well-known compounds such as GHRH, secretin, and glucagon, the activities of which have been very clearly characterized. VIP was originally isolated from the digestive tract, and has demonstrated activity in lowering blood pressure, but its role in day-to-day metabolism is still not clear.